Today a number of satellites 101 are fixedly placed on the so called geostationary path 103, see FIG. 10. Such a satellite is located on a principally fixed point above the earth surface 105, straight above a fixed point on the equator 107. These satellites send among other things TV-signals that are intended for private homes, company premises or apartments and that are usually received by paraboloid antennas placed in the direct vicinity of the place where the signal is to used for displaying television.
Paraboloid antennas intended for receiving microwave signals exist today that are arranged for simultaneously receiving signals from several satellites in the same antenna, for example the so called “space ear” which is constructed as a paraboloid mirror vertically and a circular mirror horizontally.
When one tries to capture satellite signals using a reflecting antenna, i.e. a paraboloid antenna, in a point somewhere on the earth surface, located apart from the equator, the signal sources are imaged according to optical laws that for microwaves are very complicated. Consequently a receiving device arranged for simultaneously receiving from satellites in the same antenna today has only movable receiver heads that can be individually adjusted.
Instructions on the way in which these adjustments are to be made usually accompany commercially available paraboloid antennas when buying them and they contain a number of complicated graphs and angle tables that the buyer must refer to the longitude and latitude at the place where the antenna is to be installed. Thereafter the system can be mounted by a careful setting-up according to the values of the adjustment angles obtained from these tables and graphs. Normally each receiver horn used is individually adjustable in all directions.
This fact has appeared to be unfavourable for the seller of these systems since the buyer most often cannot easily find the correct angles for receiving from each individual satellite. Few buyers succeed in making all steps in the mounting operation without assistance from well instructed persons and a number of auxiliaries for the mounting, such as signal strength indicator, compass, plummet, etc.
After one of the receiver horns has been adjusted as to its direction, it is also as a rule required that when the second horn is to be adjusted also the whole antenna must be readjusted, so that the first horn must be again adjusted and so on. The installation operation thus comprises iterative steps that certainly result in a better and better solution but is time consuming. According to the state of the art today no simple and universal method exists for easily installing an antenna for multifocus receiving, i.e. for receiving signals from different source in the same antenna.
An antenna, that refracts or images electromagnetic waves from a source having the shape of point creates a focus that is the point in which the focused signal is strongest. Also signals that arrive obliquely in relation to the optical axis of the antenna are strongly focused in a point. For antennas that are particularly designed to focus also electromagnetic wave arriving obliquely towards the antenna this effect is still more evident.
If these foci are derived for waves in directions of a large number of angles in relation to the optical axis of the antenna points are obtained forming a surface in which naturally also the focus for waves arriving along the optical axis are located. This focus surface has a shape determined by the characteristics of the antenna. For example a rotationally symmetric antenna has a rotationally symmetric focus surface about its symmetry axis that at the same time is the optical axis. For example it is for a paraboloid or an offset paraboloid antenna the axis around which the paraboloid surface is designed, i.e. is constructed around, and the focus surface has as a rule some type of cup shape.
For example a lens of waveguide character can be designed so that foci in different angles through the lens for remote signal sources form a spherical focus surface having a radius equal to the focus distance or as another extreme in a flat focus surface. One can say that the image forms a focus surface having a radius of curvature designed to have some length that is shorter than the focus distance up to and including an infinite radius and that by definition intersects the focus which is traditionally defined along the optical axis of the lens, i.e. that obtained for waves incoming alone the optical axis or from waves incoming from a source located on the optical axis.
For the rotationally symmetric lens antenna according to the published International patent application WO 94/11920 A1 or an optical photographic lens of glass the focus points form a flat surface that is rotationally symmetric in relation to the symmetry axis which at the same time is the optical axis. When imaging the geostationary path using such a lens antenna in the focus plane for such a lens a substantially straight, sometimes a little curved line, can be obtained.
Other antennas that are not rotationally symmetric have as a rule more or less an approximate rotation symmetry about the optical axis.